This invention relates to a terrestrial wavelength-division multiplexing (WDM) system in which the transmission is bidirectional along a single optical waveguide, such as a fiber.
The demand for increasing channels in optical WDM systems has created interest in bidirectional systems in which a single wave guide, such as a fiber, is used to transmit optical signals in the two opposite directions along the fiber essentially to double the number of channels that can be transmitted along the fiber. There have been two principal issues that need to be addressed in the design of such systems. First there needs to be a wavelength channel allocation plan that provides adequate isolation between channels with a minimum of overlap. To this end there needs to be provided adequate spacing in the wavelengths of adjacent channels to maintain the necessary isolation between the channels. An important consideration has been the need to avoid especially four-photon mixing (FPM) between adjacent channels traveling in the same direction, a factor which imposes a limit on the spectral density of the system, where spectral density is defined as the number of channels that can be transmitted within a unit spectral interval under essentially error-free conditions. As is known, each set of two codirectional WDM channels generates multiple new optical signals overlapping in frequency with adjacent channels, thus generating in-band crosstalk that reduces error-free transmission. The efficiency of the FPM process for generating intervening channels is directly dependent on the wavelength spacing among the WDM channels. Low FPM penalty requires wide channel spacing among WDM channels for signals traveling in the same direction. However, counterdirectionally propagating channels do not contribute significantly to the FPM process so that the spacing in an equidistant WDM grid can be halved without an observable increase in the FPM penalty if one interleaves a set of counterpropagating WPM channels. This channel structure is known in the art as an interleaved bidirectional WDM architecture and allows for spectral densities essentially double those feasible for a comparable unidirectional channel structure.
However an interleaved bidirectional WDM architecture still requires separate transmitters, receivers and compound amplifiers to provide gain in each of the two opposite directions.
A problem that arises in such an architecture is that a signal propagating in a given direction will inevitably experience factors that result in some reflection of the signal that will cause part of it to travel in a direction opposite, or counter, to its original direction of propagation and so to affect deleteriously the signals of channels launched to propagate in such opposite direction. Such energy will be described as counterpropagating or counterdirectional energy.
Accordingly, design of a bidirectional interleaved WDM system requires special consideration, particularly in the construction of the optical amplifiers of the system, since they are generally used to provide both channel amplification and channel isolation among counterpropagating sets of channels.
The present invention presents a novel approach to the isolation need of counterpropagating reflected energy in such bidirectional WDM systems.
The invention provides novel forms of optical amplifier architecture to neutralize counterpropagating signals. More particularly, the invention involves inserting along the light wave paths suppression filters of appropriate spectral form, to be termed interleavers, to selectively pass in a given direction only one of the two sets of interleaved channels. In a preferred form, the interleaver is a four-port filter that passes channel signals of a first of two sets of spectrally interleaved signals that propagates in a given direction from an input port to an output port and continues the light appropriately along a path in the desired direction, but shunts counterdirectional propagating light entering the same input port to a different output port for attenuation or absorption. A device, typical of the kind that can serve as the interleaver, is the chromatic dispersion-free Fourier transform-based wavelength splitter described in a paper entitled xe2x80x9cChromatic dispersion free Fourier transform-based wavelength splitters for D-WDMxe2x80x9d that was published in the Fifth Optoelectronics and Communications Conference IDECC 2001 Technical Digest, July 2000, pp. 374-375. Various arrangements will be described of particular design to suppress selectively counterpropagating light arising from reflections along the prescribed wave path.
In particular, a feature of the invention is a gain block for use in a WDM transmission system in which a first of two sets of optical channels of interleaved wavelengths propagates along a waveguide in one direction with low loss selectively and the second set of optical channels propagates along the same guide with low loss selectively in the direction opposite to the first direction. A characteristic of gain blocks in accordance with the invention is the inclusion of interleaver elements that are basically four-port elements is that the port at which a signal exits is a function both of the port at which it enters and the wavelength of the signal. By such inclusion there is substantially reduced the effect of reflections in the system that give rise to spurious signals that will be described as counterdirectional propagating signals, and that are of the wavelengths to be controlled by the interleaver.
The invention will be better understood from the following more detailed description taken in conjunction with the accompanying drawing.